A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon, modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50%). Additionally, as voltage or current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on a load microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as 12 volts, supplied by an input voltage source to a lower, regulated, output voltage, such as 2.5 volts, to power a load. To provide the voltage conversion and regulation functions, the power converters include active power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor or the primary winding of a transformer to the voltage source at a switching frequency that may be on the order of 100 kHz or higher.
A conventional way to regulate an output characteristic of a switched-mode power converter, such as output voltage, is to sense a current in an inductive circuit element such as an output inductor in a forward converter topology or a transformer primary winding in a forward or flyback converter topology, and compare the sensed current with a threshold current level, generally using a comparator, to control a duty cycle of the power converter. The threshold current level is generally set by an error amplifier coupled to a circuit node such as an output terminal of the power converter to regulate the output characteristic. Such a process is generally referred to as current mode control. Alternatively, the output of the error amplifier can be compared to a fixed sawtooth waveform to regulate the output characteristic. Such a process is generally referred to as voltage mode control. The mechanism to control duty cycle is a signal produced by the comparison process to turn a power switch “on” or “off.”
In dc-dc power converter applications, including point-of-load applications (commonly referred to as “PoL” applications), there is often the need to drive several power trains with paralleled outputs. A power train refers to a power-processing portion of a power converter and generally includes an active power switch, such as a MOSFET, and a reactive circuit element, such as an inductor or a transformer. Such a multiphase circuit structure helps to increase the output power. A multiphase circuit structure may also be employed to reduce EMI, to improve cooling, and to reduce the input ripple current compared to a larger single power converter.
In a power converter application wherein several power trains are coupled together with paralleled outputs, each power train has to “know” how many power trains are connected together to determine if its individual load current is low or high compared to the individual load currents provided by the other paralleled power trains. It is difficult in such an analog circuit arrangement to consistently handle information describing the number of power trains actively operating in parallel, especially if the number of active power trains can vary between a full load and a low load operating condition, which is often the case when a power converter formed with a plurality of power trains is constructed to optimize its net efficiency by selectively disabling individual power trains in response to sensing the total load current. Moreover, the accuracy with which current is shared among the active, paralleled power trains depends on the accuracy of the sensing arrangement and the data exchange mechanism that are employed to signal the total power converter load current to the individual power trains.
Another difficulty in such current sharing arrangements is the reaction of the different power trains after detection of an asymmetry in output currents from the individual power trains. From a control point of view, it is often important that adaptation of power train parameters due to a change in current sharing or a change in total load current does not interfere with load regulation, such as control of the output voltage of the power converter. An example of a change in current sharing is disabling or re-enabling of a power train after a reduction or an increase in load current. If the power trains update their parameters at different points in time, intermediate changes in power train current sharing can occur that can lead to an undesired control response, even to an instability or to a limit cycle.
Thus, there is a need for a process and related method to provide a measure of total load current information to individual power trains in a switched-mode power converter, as well as the number of power trains that are active in the power converter to enable dynamic control of current sharing by the individual power trains. This control information would advantageously be provided with a minimal number of pins that avoids the disadvantages of conventional approaches.